Aviation glossary

Above ground level, or AGL, is an altitude measured from the surface directly below an aircraft. Learn how it differs from MSL.

An automated weather observing system, or AWOS, reports airport weather such as wind, visibility, temperature, dew point, and pressure.

Automatic terminal information service, or ATIS, is a recorded airport broadcast of current weather and operational information.

Bearing is the direction from one point to another, expressed as an angle from a stated reference such as magnetic or true north.

Broken clouds are a cloud layer covering five eighths to seven eighths of the sky, coded BKN in aviation weather reports.

A ceiling is the height of the lowest broken, overcast, or obscuring cloud layer reported from an airport observation.

Class B airspace is controlled airspace around the busiest airports, normally shown as layered shelves and requiring an explicit ATC clearance to enter.

Class C airspace is controlled airspace around airports with radar and tower services, organized into a surface area and an outer shelf.

Class D airspace is controlled airspace around an airport with an operating control tower, normally extending from the surface to a published ceiling.

Class E airspace is controlled airspace that is not Class A, B, C, or D. Its floor and purpose vary by location and chart depiction.

The common traffic advisory frequency, or CTAF, is the frequency used to coordinate traffic at an airport without an operating control tower.

The crosswind component is the portion of wind blowing across a runway. Learn how it is calculated and why runway angle matters.

Density altitude is pressure altitude corrected for nonstandard temperature. Learn how it affects aircraft performance, with a worked aviation example.

Dew point is the temperature to which air must be cooled for saturation to occur at its current pressure and moisture content.

Groundspeed is an aircraft's speed across the ground. It combines the aircraft's true airspeed with the effect of wind.

The headwind component is the portion of wind blowing opposite an aircraft's direction of travel or runway heading.

Indicated airspeed is the speed shown on the airspeed indicator from pitot and static pressure. Learn what it measures and why it matters.

Instrument flight rules, or IFR, are operating rules that allow flight by reference to instruments under an approved clearance and procedure.

Magnetic variation is the angle between true north and magnetic north at a particular location and date.

Mean sea level, or MSL, is the reference used for elevations and most aircraft altitudes. Learn how MSL differs from AGL.

A METAR is a coded aviation weather observation issued at an airport. Learn how to read wind, visibility, clouds, temperature, and pressure.

A military operations area, or MOA, is designated airspace used to separate certain military training activities from other IFR traffic.

A NOTAM is a time-sensitive notice about a condition that affects an aviation facility, service, procedure, or hazard.

Overcast clouds are a cloud layer covering seven eighths to eight eighths of the sky, coded OVC in aviation weather reports.

Pressure altitude is altitude referenced to the standard pressure setting of 29.92 inches of mercury. Learn how pilots calculate and use it.

A prohibited area is airspace established to protect a location by prohibiting aircraft flight through the area.

A restricted area is airspace where flight is not wholly prohibited but is limited because of a known hazard or special activity.

Special use airspace is designated airspace where activities or restrictions require pilots to use specific operating information and procedures.

Squawk 7500 is the transponder code associated with unlawful interference or hijacking of an aircraft.

Squawk 7600 is the transponder code associated with a two-way radio communication failure.

Squawk 7700 is the transponder code used to indicate an emergency when no more specific code or instruction applies.

The standard atmosphere is a reference model of pressure, temperature, density, and altitude used for aviation calculations and performance data.

A TAF is an airport weather forecast covering a defined terminal area and validity period. Learn how to read forecast groups and times.

The tailwind component is the portion of wind blowing in the same direction as an aircraft's travel or runway heading.

Temperature lapse rate is the rate at which atmospheric temperature changes with altitude. It is used in standard atmosphere and weather analysis.

Track is the direction an aircraft moves across the ground, measured from a reference such as magnetic or true north.

True airspeed is an aircraft's actual speed through the surrounding air mass. Learn how it differs from indicated airspeed and groundspeed.

True heading is the direction an aircraft nose points measured clockwise from true north, before applying magnetic variation or compass deviation.

VA is the published maneuvering speed below which full or abrupt control inputs are less likely to cause structural damage from a single input.

Visibility is the greatest distance at which an object can be seen and recognized, reported using the applicable aviation weather convention.

Visual flight rules, or VFR, are operating rules that require pilots to remain clear of clouds and maintain visual reference as specified.

VNE is the maximum indicated airspeed that must not be exceeded. The red line and aircraft POH or AFM provide the controlling limit.

VX is the indicated airspeed that produces the greatest altitude gain over the shortest horizontal distance. The approved POH value controls.

VY is the indicated airspeed that produces the greatest altitude gain over time. The aircraft POH or AFM provides the applicable value.

A warning area is airspace extending from three nautical miles off the US coast that warns pilots of potentially hazardous activity.

Wind shear is a change in wind speed, direction, or both over a short distance. It can affect aircraft performance and flight path.